The present invention relates to a method and apparatus for measuring a three-dimensional curved surface shape in a non-contact manner.
The measurement of three-dimensional curved surface shapes is conceivable for practical use in a wide range of applications such as a three-dimensional CAD input, robot vision, body contour measurement for medical and dressmaking purposes and various techniques have been proposed in the past.
Particularly, one such technique well known as a light-section method or optical cutting method is described for example on pages 398 and 399 of "Handbook for Image Processing" (Shokodo Co., Ltd.). As shown in FIG. 1 of the accompanying drawings, this method notes a phenomemnon that when a slit Light 53 is projected from a slit light source 52 onto an object 51 to be measured, a light beam pattern formed on the surface of the object corresponds to the cross-sectional shape of the object 51 at the slit light projection position when the light beam pattern is observed from a direction different from the projecting direction and the method has been used widely by virtue of its simplicity, non-contactness and quantitative property.
When measuring the shape of a three-dimensional curved surface by this optical cutting method, while moving the slit light 53 in the directions of an arrow 54, the resulting light beam patterns are observed by a television camera (55) and the thus generated video signal is processed from moment to moment, thereby extracting (56) the beam cutting lines (the shapes of the light beam patterns) within the image and reconstructing (57) the cutting lines to construct the curved surface shape.
While a method is known in which the construction of an optical system includes a light spot scanner as a light source in place of the slit light source 52 and a high-speed light spot position detecting device known as a PSD (position sensitive detector) sensor, for example, is used in place of the television camera 55, the basic principle is the same as the one shown in FIG. 1.
While the above-mentional optical cutting method is one having various advantages, in order to detect and specify the respective points on an object to be measured, the process of extracting the beam cutting lines within each image is essential and this causes the following problems from the standpoint of measuring accuracy or reliability.
(1) Deterioration of the Measuring Accuracy and Spatial Resolution Dependent on the Shape of an Object to be Measured
With the optical cutting method, as shown in FIG. 2(a), if the surface of the object 51 to be measured in an inclined surface of an angle close to a right angle with respect to the optical axis of the slit light 53, the width w of a light beam pattern at the object surface is narrow and it is possible to make a highly accurate measurement. However, if the surface of an object to be measured is an inclined surface of an angle approximately parallel to the optical axis of the slit light 53 as shown in FIG. 2(b), the width w of a light beam pattern at the object surface is increased so that not only is the uncertainty of the position during the beam cutting line increased with resulting deteriorated accuracy, but also the amount of movement of the light beam pattern on the object surface due to the movement of the slit light source 53 is increased, thereby simultaneouly deteriorating the spatial measuring resolution.
(2) Deterioration of the Measuring Reliability Due to the Surface Reflectance of an Object to be Measured
With the optical cutting method, it is presupposed that during the process of extracting the beam cutting lines in a picture, the light beam pattern is sufficiently bright as compared with the ambient brightness so that if, for example, there is a considerable nonuniformity in the reflectance of the object surface or alternatively the angle of the inclined surface of the object surface is close to the optical axis of the slit light, thus decreasing it reflected light intensity, there are frequent cases where during the extraction of the beam cutting lines the occurrence of discrete points is frequently caused or entirely wrong points are detected by mistaking them for the beam cutting lines. Such a phenomenon is caused in cases where there exists any background light other than the slit light during the measurement, and each of such phenomena causes deterioration of the measuring reliability or restricts the measuring environments of objects to be measured.
As a result, due to some measuring problems arising from the beam cutting line extraction process, the optical cutting method has many restrictions from the application standpoint e.g., the shapes, the surface contours and the environments of objects to be measured and it applications are limited for its advantages including the simplicity, non-contactness, quantitativeness, etc. Thus, this method has not gone so far as to be assembled as a general-purpose three-dimensional curved surface shape measuring apparatus and put widely into practical use.